Much of the recent decrease in violence in Iraq has been due to the co-opting of much of the Sunni/Ba'athist opposition. Besides getting the opportunity to chase Al-Qaeda out of the neighborhoods, the Sunnis have received direct monies and weapons from the United States, and permission to receive more from sources like Saudi Arabia and the Gulf States.
I saw a recent estimate that put the American aid, much of it military, at 250 million dollars.

This makes sense in the short term, as resistance to the occupation drops and the Sunnis feel more secure in a Shia-dominated Iraq. However, I truly doubt that the Sunnis will never use their re-armed militias against the Shia when the power-sharing arrangement inevitably breaks down after U.S. forces have left Iraq. In effect, we are guaranteeing that there will eventually be a large scale armed conflict or even a civil war, as the Shia have several well-armed militias of their own. The army will be of no help, as only one brigade is a mixed one, the rest being only Kurd, only Sunni Arab or only Shia Arab.

This strikes me as typical of all our thinking on Iraq: short-term gain for long-term pain.